In a known common rail fuel injector, a valve needle is engageable with a seating to control the delivery of fuel to a combustion space. The position of the needle is controlled by controlling the fuel pressure within a control chamber. In such an arrangement, in the event that the valve needle becomes stuck in a lifted position, fuel will be delivered continuously by the injector. Such a continuous discharge of fuel under pressure could cause a catastrophic failure of the engine and/or fuel system.
An alternative arrangement comprises a valve needle spring biased towards a seating, and a control valve controlling the supply of fuel to a delivery chamber of the injector. In such an arrangement, if the control valve sticks in an open position, fuel delivery will occur continuously and may result in failure as described hereinbefore.
It is an object of the invention to provide an injector in which the disadvantages described hereinbefore are obviated or mitigated.